Low battery gauge reading

I have a 2007 GMC Envoy SLT 4x4 Sony battery gauge should read 14.4v in a normal situation what’s happening is when I start the car it shoots up to 15v then settles at 14,4 after a few aecnonds .Then after about 5 or 10 min the gauge starts to drop settleing right about 12v even when the motor is at higher rpm it does not change the gauge once it drops to 12v once this happens I notice my power windows are a little slugish and my interior lights are more dim then normal. When I restart the car the process repeats and everything works fine when the gauge is at 14.4 . I’m a lic electrician so I did some troubleshooting checked all connections with no luck . The battery is less than 6 months old . I brought it to auto part store where they put a testing equipment on it . It came back voltage regulator failure . I’m this model the voltage regulator is inside the alternator as one unit . So I replaced the alternator and got the same results . Any idea as to what the problem could be because I’ve been racking my brain all week over this all week also I took the battery connections off checked them and then made sure they were torqued properly . And if it matters my batter has not died I’ve had no trouble starting the car . ? Any ideas ??